Effect of malting and fermentation on colour, thermal properties, functional groups and crystallinity level of flours from pearl millet (Pennisetum glaucum) and sorghum (Sorghum bicolor)

The effect of malting and fermentation on colour, thermal properties, level of crystallinity and functional groups of Agrigreen, Babala pearl millet cultivars and sorghum flours were studied using response surface methodology. The central composite rotatable design was performed on two independent variables in terms of malting and fermentation time at intervals of 24, 48 and 72 h, respectively using design expert software. One-way analysis of variance at p < 0.05, regression analysis, response surface plots for interactions between malting and fermentation processing times with response variables were recorded. The results indicated that malting and fermentation times have significant effects on the thermal and colour properties as well as the level of crystallinity and functional groups of pearl millet cultivars and sorghum flours. In terms of colour, sorghum exhibited high content in L∗ at 72.02–73.72, a∗ ranged from 2.50-3.30 and chrome at 13.10–14.82, while Babala flour was high in b∗ and hue at 12.15–14.27 and 73.00–84.80, respectively. In terms of thermal properties, sorghum was noticed to be high in melting peak at 87.57–104.83 °C, 102.66–111.14 °C for end completion and gelatinisation range at 10.70–25.79 °C, whereas, Babala recorded high values in onset and enthalpy at 93.20–100.11 and 5.72–21.62 J/g, respectively. The Fourier transform infrared (FTIR) spectroscopy showed that malted and fermented Agrigreen, Babala and sorghum flour showed peaks in OH, carbonyl, amide and C–O bonding. The optimal processing time for the colour of Agrigreen was 50.69 h (malting) and 39.38 h (fermentation), Babala was 54.40 h (malting) and 65.30 h (fermentation); and sorghum was 49.90 h (malting) and 54.61 h (fermentation). While the optimal malting and fermentation time for thermal properties for Agrigreen was 45.78 h and 42.60 h; Babala was 40.94 h and 29.07 h and sorghum was 34.83 h and 36.33 h, respectively with product quality at the desirability of 1.00. X-ray diffractogram results of the optimum processing points of the thermal properties showed that malted and fermented Agrigreen, Babala and sorghum flour showed high peak intensities, while the unprocessed flour exhibited diffused peaks. The obtained results would assist food processing companies to improve the colour and thermal properties and also the behaviour of the crystallinity and functional groups in food during processing.